Punta Sur - Devil's Throat

A hole in the coral, unremarkable until you are directly over it, marks the entrance. It sits at the edge of a sand plateau 24 to 27 m down, beside the Punta Sur wall, and groups settle on the sand and sort out their trim before anyone goes near it.

Then you go in single file behind the guide, spaced, into a shaft that drops at roughly 45 degrees and narrows to about a metre and a half in one section. It darkens fast, and then the blue light at the far end appears. The whole passage takes 60 to 90 seconds. Red and orange sponge coats the walls, and at this depth daylight has no red left in it, so the colour exists only inside a light beam. Frog kick is the technique every briefing agrees on, because a flutter kick in a passage this narrow silts out everyone behind you. Divers describe the right body position both ways, feet first and head first, so take the one the guide briefs on the day.

Then the exit, which is the dive. You come out of the reef into open water between 35 and 43 m, most often given as 40, on a sheer wall with the drop falling away beneath you and the surface a long way up. It lasts about a minute before the computer starts asking for the ascent.

The ascent differs by operator. One route threads shallower swim-throughs back up through the same mound and finishes over a sandy slope near 10 m, and another comes back along the shallower sections of the reef to a stop near the top. Southern stingrays and resting nurse sharks are reported on the sandy stretches, and the passages above the tunnel are, by several accounts, the better part of the coral head.

Listings that give a maximum around 27 m are describing the mouth of the tunnel, and the dive carries on down through it. Divers' own logs put the deepest point anywhere from about 115 to 140 ft, and that spread has mechanical causes. Several passages run through the coral head at different depths, so the route your group takes changes where you come out. The exit is also where people wait, hovering in open water off the wall while the rest of the group files through one at a time, and the wait alone can set the deepest point of the dive. Below it all the wall keeps falling, so nothing stops a slow drift downward.

That makes the site a different proposition from a reef or a wall you can move shallower on whenever you like. Here you are in a queue in a one-way passage, at a depth where the gas goes five times faster than at the surface. Plenty of experienced local divers rate the shallower swim-throughs over the top of the same coral head above the tunnel itself, and some skip the tunnel by choice and meet the guide below the exit. Doing so is briefed and ordinary.

Operators set the bar here, and it sits above the card. They ask for Advanced Open Water and most want to have dived with you already and watched your buoyancy and consumption. The boat has to agree as well: everyone aboard has to want the dive and be able to do it, or the group goes somewhere else.

The park's rules add their own limits at this site. An authorised guide supervises every recreational dive1, groups are capped at six divers per guide, and night diving and anchoring are prohibited in this zone1. Ask your dive shop which reefs are resting for your dates, since the park can close this complex.

Leave nitrox alone unless the shop can blend for the profile. EAN32 tops out at 33.8 m on a PO2 of 1.4, roughly six metres above the usual exit.

Operators list a dive light as required, and the sponge colour is the reason to carry a proper primary rather than a backup in a pocket. Gas is what people underestimate, so work out beforehand what it takes to leave the overhead and reach the surface sharing air, and ask which cylinder you will be handed. Ask the group size too: a queue of twelve at 27 m spends your gas before you enter the tunnel.

At the exit, keep your eyes on the gauge. Divers report drifting to around 120 ft (37 m) while the group finishes the passage, because the wall below gives nothing to hold a depth against.